Silver catalyst for production of ethylene oxide from ethylene

ABSTRACT

A catalyst for the production of ethylene oxide from ethylene, said catalyst comprising a porous carrier composed of a molded article of a refractory material and at least silver grains deposited on the carrier, wherein 
     (A) silver is distributed on the outside surface of the carrier and on the inner surfaces of the pores of the carrier, 
     (B) silver grains distributed on the inner surfaces of the pores of the carrier have an average diameter of 0.05 to 0.4 micron, and 
     (C) the loading (S) of silver on the outside surface layer of the catalyst and the loading (I) of silver on the innermost layer of the catalyst satisfy the following expression 
     
         I≧0.65S; 
    
      and a process for producing a catalyst for the production of ethylene oxide from ethylene, which comprises impregnating an aqueous solution containing a silver salt and an amine as a complex forming agent in a porous carrier composed of a molded article of a refractory material, and heating the carrier with superheated steam to deposit silver on the carrier.

This application is a continuation of Ser. No. 767,831 filed Aug. 20,1985 now U.S. Pat. No. 4,690,913.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel silver catalyst for producing ethyleneoxide by the oxidation of ethylene with molecular oxygen, and a processfor producing the catalyst.

2. Description of the Prior Art

A method known for preparing a silver catalyst for use in the oxidationof ethylene with molecular oxygen comprises impregnating a porousrefractory carrier with an aqueous solution of a silver salt complexedwith an amine (encompassing ammonia), and heating the impregnatedcarrier with air or the like to deposit silver on the carrier. By usingamines, the silver salt decomposed and reduced at low temperatures isconverted into an amine complex, and thus can be formed into a uniformaqueous solution. Hence, fine and uniform silver grains can be depositedon a porous carrier composed of a molded article of a refractorymaterial to give a catalyst of excellent performance.

According to the description of Japanese Patent Publication No.22146/1980, to deposit silver from a silver salt-containing aqueoussolution and make a catalyst, heating at 100° to 375° C. for 2 to 8hours is required, and air is used as a heating medium. Investigationsof the present inventors have shown that the above method of heatingresults in a non-uniform distribution in the amount of deposited silverwithin a catalyst particle, and when the heating temperature and timedescribed in the Examples of the above-cited Japanese Patent Publicationare employed, the silver grains agglomerate and grow. For example, ithas been ascertained that in a catalyst having 13% by weight of Agdeposited on a carrier having a surface area of 0.4 m² /g which isprepared by calcination in air at 270° C. for 2 hours, many of thedeposited silver grains have a particle diameter of at least 0.4 micron,and thus, the silver grains are large and are distributed non-uniformlyin each catalyst particle. For this reason, such a catalyst cannotexhibit a sufficient catalytic performance.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel catalyst for theproduction of ethylene oxide from ethylene, in which silver grainsdeposited on a carrier are fine and uniform to give higher catalyticactivity, and the amount of the silver grains deposited is very uniformfrom the surface layer to the inner layer of each catalyst particle, andthe speed of agglomeration of the silver particles that occurs as thereaction proceeds is slow to give a long catalyst life, and a processfor producing the novel catalyst.

According to this invention, there is provided a catalyst for theproduction of ethylene oxide from ethylene, said catalyst comprising aporous carrier composed of a molded article of a refractory material andat least silver grains deposited on the carrier, wherein

(A) silver is distributed on the outside surface of the carrier and onthe inner surfaces of the pores of the carrier,

(B) silver grains distributed on the inner surfaces of the pores of thecarrier have an average diameter of 0.05 to 0.4 micron, and

(C) the loading (S) of silver on the outside surface layer of thecatalyst and the loading (I) of silver on the innermost layer of thecatalyst satisfy the following expression

    I≧0.65S.

According to this invention, there is also provided a process forproducing a catalyst for the production of ethylene oxide from ethylene,which comprises impregnating an aqueous solution containing a silversalt and an amine as a complex forming agent in a porous carriercomposed of a molded article of a refractory material, and heating thecarrier with superheated steam to deposit silver on the carrier.

The process of this invention has the advantage that the catalyst havingthe above characteristic features and being free from the defects of theprior art can be produced particularly at low temperatures within shortperiods of time.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 shows temperature elevation patterns of the catalyst of thisinvention (Example 1) and a control catalyst obtained by heating in air(Comparative Example 1); FIGS. 2-A to 2-D are scanning electronmicrographs (magnification 10,000×), FIG. 2-A showing the porous surfaceof the interior of a catalyst particle in the catalyst of this invention(Example 1), FIGS. 2-B and 2-C respectively showing the outside surfacelayer and the surface of the pores inside of a catalyst particle in thecontrol catalyst (Comparative Example 1), and FIG. 2-D showing theporous surface of the inside of a catalyst particle in a controlcatalyst (Comparative Example 2); and FIG. 3 shows the distribution ofsilver in a catalyst particle in the catalyst of this invention(Example 1) and the control catalyst (Comparative Example 1).

DETAILED DESCRIPTION OF THE INVENTION

The catalyst of this invention for the production of ethylene oxide fromethylene, which has the aforesaid characteristics and advantages, can beproduced by impregnating an aqueous solution containing a silver saltand an amine as a complex forming agent in a porous carrier composed ofa molded article of a refractory material, and heating the carrier withsuperheated steam to deposit silver on the carrier.

In the process of this invention, silver is deposited on the carrierpreferably by bringing the porous carrier impregnated with the aqueoussolution containing a silver salt and an amine as a complex formingagent into contact with superheated steam at a temperature of at least120° C. while it contains at least a part of the aqueous solution.Particularly, it is advantageous that silver is deposited on the carrierby contacting the porous carrier impregnated with the aqueous solutioncontaining a silver salt and an amine as a complex forming agent, withsuperheated steam at 120° to 500° C., preferably 120° to 300° C., aboveall 150° to 260° C., while the ratio of removal of the aqueous medium inthe aqueous solution is 0 to 70% by weight, preferably 0 to 50% byweight. That is, it is particularly advantageous that heating of theimpregnated carrier with the superheated steam be conducted under thecondition that at least 30% by weight, preferably at least 50% byweight, of the aqueous medium of the aqueous solution remains with thecarrier, to at least substantially dry the aqueous medium and decomposethe silver salt to deposit silver on the carrier.

The silver salt that can be used in the process of this invention may beany silver salt which when formed into a complex soluble in an aqueousmedium with the amine (including ammonia) decomposes at a temperature ofnot more than 500° C., preferably not more than 300° C., especiallypreferably not more than 260° C., to deposit silver. Examples of such asilver salt are silver oxide, silver nitrate, silver carbonate andsilver carboxylates such as silver oxalate and silver acetate. Thesilver carboxylates are preferred.

The amine as the complex forming agent may be any amine which acts as aligand for maintaining silver in solution. Examples include pyridine,acetonitrile, ammonia and primary or secondary amines having 1 to 6carbon atoms. Examples of preferred amines are ammonia, monoamines suchas pyridine and butylamine, alkanolamines such as ethanolamine, alkylenediamines having 2 to 4 carbon atoms, and polyamines. Diamines having 2to 4 carbon atoms are especially preferred, and ethylenediamine and1,3-propanediamine are suitable. A combination of ethylenediamine and1,3-propanediamine is most preferred. The combined use of the amine withanother amine or another compound, for example, a tiny amount ofdimethylformamide is also effective. The silver salt and the amine areformed into a uniform solution, preferably a uniform aqueous solution.Water-miscible organic solvents such as alcohol, or a mixture thereofwith water may also be used to prepare the solution. The resultingsolution is impregnated into the porous refractory carrier.

Examples of the porous refractory carrier are alpha-alumina, siliconcarbide, titania, zirconia and magnesia. An alpha-alumina carrier havinga surface area of 0.01 to 2 m² /g, preferably 0.2 to 1.2 m² /g,particularly 0.2 to 0.7 m² /g, a pore volume of 0.2 to 0.5 ml/g and anaverage pore diameter of 0.1 to 20 microns is preferred. The porousrefractory carrier is in the form of a molded article of the refractorymaterial having a size of about 4 to 15 mm which is, for example,spherical, ring-like, or cylindrical.

The impregnating operation is carried out by methods known to thoseskilled in the art. As required, such operations as pressure reduction,heating, and rotation spraying, and devices therefor are used. Theconcentration of silver and the amount of the amine in the impregnatingsolution are adjusted so that the amount of silver deposited becomes 5to 20% by weight based on the finished catalyst. The amine is added inan amount sufficient to complex the silver salt (usually, two aminogroups correspond to one mole of silver). Usually, the amine is added inan amount 10 to 30% above the equivalent amount.

In the proces of this invention, the porous carrier composed of themolded article of the refractory material is impregnated with theaqueous solution containing the silver salt and the amine as a complexforming agent, and then heated over superheated steam, preferably at atemperature of at least 120° C. to substantially complete the depositionof silver on the carrier.

The process of this invention is therefore clearly different from theprocess for producing a catalyst for the production of ethylene oxidedisclosed in Japanese Laid-Open Patent Publication No. 1191/1978, whichcomprises impregnating a carrier with an aqueous solution of a thermallydecomposable silver complex compound, a sodium compound and a heavyalkali metal compound, treating the carrier with steam, especiallysaturated steam, until the silver compound begins to decompose with caretaken not to cause a loss of water from the impregnated carrier, andthen heating the carrier in a gas such as CO₂, N₂ or air at atemperature of 150° to 300° C. until the carrier attains a constantweight.

In the present invention, the deposition of silver from an impregnatingaqueous solution of silver oxalate complexed with 1,3-propanediaminetakes place at a temperature in the vicinity of 120° C. If only theamount of heat required to complete the silver deposition reaction issupplied from superheated steam, the catalyst can be prepared by heatingwith superheated steam at that temperature.

Agglomeration of silver deposited on the carrier occurs vigorouslyparticularly when the heating is done in air at a high temperature ofmore than 200° C. The heating of the impregnated carrier in superheatedsteam in the present invention offers the advantage that theagglomeration of the deposited silver grains is very effectivelyinhibited.

However, in superheated steam, too, the silver grains tend to beagglomerated to a greater extent as the temperature of the superheatedsteam becomes higher beyond 260° C. or the heating time becomes longer.It is preferred therefore to control the conditions so as to avoidagglomeration.

Advantageously, the amount of silver deposited is 5 to 20% by weight,preferably 8 to 15% by weight, based on the entire catalyst.

It has been found in accordance with this invention that the diameter(the shorter diameter when the grains are not spherical) of typicalsilver grains deposited on the carrier depends mainly upon the ratio ofAg deposited and the surface area of the carrier. It can be said thatwhen in the present invention, the ratio of Ag deposited is in apreferred range of 8 to 15% and the surface area of the carrier is in arange of 0.2 to 0.6 m² /g, the diameter, d (microns), of the silvergrains is nearly proportional to [the amount of Ag deposited]⁰.2multiplied by [the surface area of the carrier]⁻¹.2.

Japanese Patent Publication No. 33565/1978 discloses a process forproducing a catalyst which comprises (a) impregnating a thermallydecomposable silver salt in an inert particulate support, (b) drying itat a temperature not exceeding 160° C., (c) passing superheated steam sothat its temperature reaches a selected point within a range of 270° to350° C., (d) substituting air for the steam at the aforesaid temperatureover at least 1 hour, and (e) passing heated air for at least 30 minutesat the same temperature. This process requires a very complex andtime-consuming heat-treatment during the catalyst preparation, and onlyafter this heat-treatment, a good catalyst can be obtained.

The above-cited Japanese patent document does not describe the use of asilver salt complexed with an amine as the thermally decomposable silversalt. Accordingly, from the standpoint of using the amine complex in thepresent invention, the process described in the Japanese patent documentemploys the undesirable high temperature, long time heat-treatment. Forexample, while the heat-treatment in accordance with this invention iscarried out at a temperature in the range of 150° to 260° C. for aperiod of as short as about 15 minutes, the process described in theJapanese patent document requires the superheated steam treatment at270° to 350° C., preferably 290° to 320° C., and the heated airtreatment for at least 1.5 hours. If the amine complex of the silversalt used in this invention is subjected to this treatment, thedeposited silver grains are remarkably agglomerated and distributednon-uniformly within the catalyst particle. Presumably, theagglomeration is due mainly to the high-temperature air treatment, andthe non-uniform distribution is due mainly to the large temperaturedifference between the temperature (about 100° C.) of the impregnatedcarrier and the temperature of the superheated steam at the time ofevaporation of water (because of this temperature difference,evaporation of water occurs too vigorously). In addition, in the processof the Japanese patent document, the operation of drying the impregnatedcarrier in the dry state for a sufficient long period of time (1 to 10hours) is essential prior to the heat-treatment with superheated steam.This method of treatment is quite different from the heating of theimpregnated carrier in the present invention with superheated steamwhile the impregnated carrier still contains water.

It is preferred that in the process of this invention, at least oneelement as a cationic component (D) selected from the group consistingof (D-1) lithium, sodium, potassium, rubidium and cesium (alkali metalelements), (D-2) calcium and barium (alkaline earth metal elements), and(D-3) thallium, tin and antimony is deposited on the porous carrier inaddition to the silver grains.

As the cationic component, (i) at least one alkali metal elementselected from the group consisting of lithium, sodium, potassium,rubidium and cesium, or (ii) a combination of at least one alkali metalelement (i) with barium is preferred.

As the cationic component of group (i), a combination of sodium andcesium, a combination of sodium and potassium, a combination of sodiumand rubidium, a combination of potassium and cesium, and a combinationof lithium and cesium are preferred. As the group (ii), a combination ofthe two elements mentioned above with barium, especially a combinationof sodium, cesium and barium, is preferred.

In the process of the present invention, it is more preferred to depositat least one element as (D) a cationic component selected from the groupconsisting of (D-1) at least one alkali metal element selected from thegroup consisting of lithium, sodium, potassium, rubidium and cesium,(D-2) at least one alkaline earth metal element selected from calciumand barium, and (D-3) at least one element of thallium, tin andantimony, and at least one element as an anionic component (E) selectedfrom the group consisting of fluorine, chlorine and bromine on theporous carrier in addition to the silver grains.

To deposit the cationic component on the porous carrier, it is generallydesirable to add it in the form of a water-soluble compound, generally anitrate, halide, hydroxide, carbonate, bicarbonate or carboxylate. Anoxide may also be used. Specific examples of the water-soluble compoundof the cationic component are lithium carbonate, sodium carbonate,sodium bicarbonate, sodium acetate, potassium nitrate, cesium nitrate,cesium chloride, rubidium nitrate, barium nitrate, barium hydroxide,barium oxide, calcium hydroxide, calcium oxide, thallium chloride, tinbromide and antimony chloride.

Suitable amounts of the above compounds may be added to the impregnatingsolution at the same time or individually to deposit them on the porouscarrier. These additional compounds may be deposited on the carrierbefore or after the deposition of silver. Deposition of these cationiccomponents by heating may also be carried out by known methods. But theuse of superheated steam is preferred because the cationic component isdistributed uniformly in the catalyst particles and this favors theperformance of the catalyst.

The anionic component may be added in the same way as in the case of thecationic component. Desirably, the anionic component is generally addedin the form of a water-soluble compound. It is preferred to add it inthe form of a salt with the above cationic component, that is, an alkalimetal element such as lithium, sodium, potassium, rubidium and cesium,an alkaline earth metal element such as barium or calcium, or thallium,tin or antimony. An ammonium salt may also be used. Some examples arelithium bromide, sodium chloride, potassium fluoride, cesium chloride,thallium chloride, and ammonium chloride.

A suitable amount of the above component is added to the impregnatingsolution, and the anionic compound may be deposited on the porouscarrier at the same time as the deposition of silver.

It is also possible to deposit the anionic component before or after thedeposition of silver. The deposition of the anionic component by heatingmay also be performed by known methods. But the use of superheated steamis preferred because the anionic component is distributed uniformly inthe catalyst particles and this favors the performance of the resultingcatalyst.

In the catalyst of this invention, the cationic component and theanionic component which modify silver as a reactive species and improvesthe selectivity of the reaction are uniformly distributed within eachcatalyst particle, and these components uniformly modify silver. Hence,the selectivity of ethylene oxide formation is increased.

The process of this invention thus gives a catalyst for the productionof ethylene oxide from ethylene. This catalyst comprises a porouscarrier composed of a molded article of a refractory material and atleast silver grains deposited on the carrier, wherein

(A) silver is distributed on the outside surface of the carrier and onthe inner surfaces of the pores of the carrier,

(B) silver grains distributed on the inner surfaces of the pores of thecarrier have an average diameter of 0.05 to 0.4 micron, preferably 0.1to 0.3 micron, and

(C) the loading (S) of silver on the outside surface layer of thecatalyst and the loading (I) of silver on the innermost layer of thecatalyst satisfy the following expression

    I≧0.65S, preferably I≧0.7S.

The average diameter of the silver grains distributed on the innersurfaces of the pores of the carrier in (B) above can be measured byobserving the section of the catalyst particle by a scanning electronmicroscope. For example, with regard to silver grains clearly observedby a scanning electron micrograph (for example 10,000×), the diameters(the shorter diameters when the grains are not spherical) of about 30larger silver grains and those of about 30 smaller grains are read, andthe total of the diameters read is divided (averaged) by the totalnumber (60) of the grains. This gives the average diameter.

The loading (S) of silver on the outer surface layer of the catalyst ofthis invention and the loading (I) of silver on the innermost layer ofthe catalyst defined in (C) above can be determined by gradually shavingoff the catalyst from the outer surface to the inner layer of thecatalyst of this invention, and measuring the content (weight) of silverper unit weight (for example, 1 gram) of the catalyst so shaven.

In the present invention, the outside surface layer of the catalystdenotes a portion having a weight of about 5% by weight (in the range ofabout 4-6%) shaven as uniformly as possible from the outside surface ofone catalyst (carrier) particle toward its inner layer when the weightof one catalyst particle is taken as 100%. The innermost layer of thecatalyst denotes an inner layer (innermost layer) of the catalystparticle which is left after about 60% on an average (in the range ofabout 50 to 70%, preferably 55 to 65%) has been shaven off from theoutside surface of the catalyst particle (carrier) toward its innerlayer as uniformly as possible.

A simple method of measuring S and I is to take 30 to 50 catalystparticles, measuring their entire weight, rotating the particles in arotating vessel to shave off the catalyst particles from the surfacetoward the inside layer, and determine S and I as average values of thecatalyst particles in accordance with the above method.

In the catalyst of this invention, the following relation is satisfiedbetween the loading (S) of silver of the outside surface layer of thecatalyst and the loading (I) of silver of the innermost layer of thecatalyst.

    I≧0.65S,

preferably

    I≧0.7S.

In the formula I≧0.65S, I is preferably larger than 0.65S.

It is evident therefore that in the catalyst of this invention, thesilver grains are very uniformly deposited ranging from the surfacelayer of the catalyst particle toward its innermost layer.

It is evident from the average diameter specified in (B) above that inthe catalyst of this invention, the silver grains distributed on theinner surfaces of the pores of the catalyst carrier are very fine anduniform and do not substantially contain large agglomerated masses.

The silver grains deposited on the carrier in this invention have anaverage particle diameter of preferably as fine as not more than 0.3micron, especially not more than 0.2 micron, and therefore, the catalysthas high activity. Furthermore, since the distribution of the silvergrains in the catalyst particles is very uniform as typically shown byI≧0.7S, preferably I>0.75S, the speed of agglomeration of the silvergrains which occurs as the reaction proceeds is slow, and the activelifetime of the catalyst is prolonged. In addition, in preferredembodiments, the cation component and the anionic component which modifysilver as a reactive species and improve the selectivity of the reactionare used.

Thus, in one preferred embodiment of the present invention, there isprovided a catalyst comprising the aforesaid porous carrier composed ofa molded article of a refractory material and deposited on the carrier,not only the silver grains but also at least one element as a cationiccomponent (D) selected from the group consisting of (D-1) at least onealkali metal element selected from the group consisting of lithium,sodium, potassium, rubidium and cesium, (D-2) at least one alkalineearthmetal element selected from calcium and barium, and (D-3) at leastone element of thallium, tin and antimony.

The especially preferred cationic component (D) is

(i) at least one alkali metal element (D-1) selected from the groupconsisting of lithium, sodium, potassium, rubidium and cesium, or

(ii) a combination of at least one alkali metal element (D-1) withbarium (D-2).

According to another preferred embodiment, the catalyst of thisinvention comprises the aforesaid porous carrier composed of a moldedarticle of a refractory material and deposited on the carrier not onlythe silver grains but also at least one element as a cationic component(D) selected from the group consisting of (D-1) at least one alkalimetal element selected from the group consisting of lithium, sodium,potassium, rubidium and cesium, (D-2) at least one alkaline earth metalelement selected from calcium and barium, and (D-3) at least one elementof thallium, tin and antimony, and at least one element as an anioniccomponent (E) selected from the group consisting of fluorine, chlorineand bromine.

Preferably, the cationic component is at least one of lithium, sodiumand barium, and/or at least one of potassium, rubidium and cesium.

One particularly preferred catalyst of this invention is one in whichthe cationic component is deposited on the outside surface of thecarrier and on the inner surfaces of the pores of the carrier, and theloading (Sc) of the alkali metal element as the cationic component onthe inner surface of the pores in the outside surface layer of thecatalyst and the loading (Ic) of the alkali metal element as cationiccomponent on the inner surfaces of the pores in the innermost layer ofthe catalyst have the following relation.

    Ic≧0.2Sc

The Sc and Ic can be measured by the same method as the method ofmeasuring S and I with regard to the distribution of the silver grainsin the catalyst.

When at least one cationic component of lithium, sodium and barium isdeposited on the outside surface of the carrier and the inner surfacesof the pores of the carrier, the loading (Sc) of lithium and/or sodiumon the inner surfaces of the pores in the outside surface layer of thecatalyst and the loading (Ic) of lithium and/or sodium on the innersurfaces of the pores in the innermost layer of the catalyst have thefollowing relation.

    Ic≧0.3Sc, preferably Ic≧0.4Sc.

When at least one alkali metal element of potassium, rubidium and cesiumis deposited on the outside surface of the carrier and on the innersurfaces of the pores of the carrier, it is especialy advantageous thatthe loading (Sc) of the alkali metal element on the inner surfaces ofthe pores in the outside surface layer of the catalyst and the loading(Ic) of the alkali metal element on the inner surfaces of the pores inthe innermost layer of the catalyst have the following relation.

    Ic≧0.5Sc, preferably Ic≧0.6Sc.

The preferred amount of the cationic component based on the entirecatalyst is

(1) 0.1 to 1% by weight for sodium and lithium,

(2) not more than 0.1% by weight for potassium, rubidium, cesium andthallium, and

(3) not more than 1% by weight for barium.

The preferred amount of the anionic component deposited is not more than0.05% by weight.

The amount of sodium as the cationic component is preferably more than50 ppm (mg/kg of catalyst) based on the catalyst, particularly from 500ppm to 1% by weight. When the amount of sodium is too large, both theactivity and selectivity of the resulting catalyst are reduced, and ifit is too small, the dispersion of Ag grains on the carrier is worsethan in the catalyst of this invention when the catalyst is observedunder a scanning electron microscope, and consequently, the catalyticactivity is low and the effect of adding halogen is not sufficientlymanifested.

The preferred amount of cesium is smaller than sodium, and is preferably10 ppm to 0.5% by weight, more preferably 15 ppm to 0.1% by weight. Ifit is too large, the activity of the catalyst is markedly reduced, andif it is too small, the effect of halogen cannot be manifested fully.

The preferred amount of barium to be added is more than 10 ppm but notmore than 1% by weight, especially preferably 20 ppm to 3000 ppm. If itis too small, there is no effect of adding barium. If it is too large, aprecipitate may form in the impregnating solution or the impregnatingstep becomes complex. Moreover, the activity of the catalyst is lowered.

The amount of the halogen selected from fluorine, chlorine and bromineis suitably 5 ppm to 0.1% by weight, preferably 7 ppm to 0.07% byweight, based on the catalyst. If it is added in too large an amount, itshows a poisoning action and drastically reduces the performance of thecatalyst. In the present invention, by adding a minute amount ofhalogen, which shows a poisoning action when added in large amounts,with the cationic component such as sodium, cesium and barium, a silvercatalyst having an increased performance can be obtained.

When both the cationic component and the anionic component are used, asmall amount of lithium, rubidium, potassium or thallium may be added inaddition to sodium, cesium and barium.

The above catalyst ingredients are used as a supported catalyst mainlyfor economic reasons and in view of the catalyst life. A porousrefractory material is used as the carrier, and preferably has a BETsurface area of 0.1 to 5 m² /g and a pore volume of at least 0.2 ml/g. Aporous refractory material composed mainly of alpha-alumina ispreferred.

When sodium, cesium and barium as the cationic component and the halogenas the anionic component are to be deposited, the depositions of thecationic component and the anionic component may be effectedsimultaneously or separately, and in various modes in any desired stagesof the catalyst preparation. For example, the depositions of thecationic and anionic components may be performed before, during or afterthe impregnation of the silver compound. In order to secure theuniformity of the impregnating solution and to simplify the catalystpreparing process, it is most preferred to impregnate the sodiumcomponent before the deposition of the silver compound and impregnatebarium, cesium and halogen at the same time as the impregnation of thesilver compound.

According to this invention, whnn the impregnated carrier is at a lowtemperature, superheated steam rapidly raises its temperature anduniformly heats the entire carrier layer, as shown in FIG. 1. Usually inthe process of this invention for producing the catalyst, theevaporation of water and the deposition of silver by the decompositionof the silver complex take place during the heating step. The process ofthis invention is superior to the prior art in that the superheatedsteam induces the evaporation of water and the deposition of silveruniformly either simultaneously or separately. As a result, thedeposited silver grains are fine as shown in FIG. 2-A, and as shown inFIG. 3, silver and the other additive components are distributeduniformly. The catalysts produced by the present invention thereforehave high activity, good selectivity and a long lifetime.

Superheated steam having a pressure in the vicinity of normalatmospheric pressure is practically feasible and can be used as thesuperheated steam used in the present invention. It has a temperature of120° to 500° C., especially 120° to 300° C., especially preferably 150°to 260° C. The heating time is preferably about 1 minute to about 3hours, but is preferably shorter in view of the practical feasibilityand performance of the catalyst. Usually, a period of 3 minutes to 15minutes is most preferred. The shortest time required is determineddepending upon the amount of the impregnated carrier to be heated, thetemperature of the steam and the flow rate of the steam. Steam flowspeeds of 0.3 m/sec to 5 m/sec are preferred in view of the performanceand practical feasibility of the catalyst.

Heating with superheated steam in this invention may be carried out asfollows:

The impregnated carriers are laid in a single layer or stacked in amultiplicity of layers in the form of a fixed bed or a moving bed, andsuperheated steam may be passed upwardly, downwardly or sideways throughthe layer or layers. Since the entire layers can be heated at a uniformtemperature by the superheated steam, there is no non-uniformity ofsilver distribution among the layers. Thus, from a practical viewpoint,heating of multiple layers is economical. Nitrogen, air, etc. may beincluded in some amount into the superheated steam. Steam at the outletcontains amines and other decomposition products formed by thedecomposition of the silver salt, and to prevent accumulation of thesecompounds, some amount of steam should be purged. Basically, however,recycling of superheated steam is possible and economical. For example,90% of the superheated steam initially fed may be recycled although thisamount may vary depending upon the amount of the steam and the amount ofthe impregnated carrier.

It is preferred in the process of this invention that the porous carrierimpregnated with the aqueous solution containing the silver salt and theamine as a complex forming agent and optionally the cationic componentand/or theanionic component is heated with superheated steam either assuch or after removing the excess of the impregnating solution, whilethe ratio of removal of the aqueous medium from the aqueous solutionreaches 0 to 70%, preferably 0 to 50% by weight; or the impregnatedcarrier is dried for example in a current of air at a temperature of notmore than 100° C. and then heated with superheated steam by the methoddescribed above, thereby to deposit silver on the carrier.

The reaction of converting ethylene into ethylene oxide by using thecatalyst of this invention can be performed by a conventional operatingprocedure. For example, the pressure is 1 to 35 kg/cm², and thetemperature is 180° to 300° C., preferably 200° to 260° C. Theproportion of ethylene is 1 to 40% by volume, and the proportion ofoxygen is 1 to 20% by volume. Generally, it is preferred to use a fixedproportion, for example 20 to 70% by volume, of a diluent such asmethane. Oxygen is supplied in the form of air or as industrial oxygen.The addition of a reaction modifier, such as ethylene dichloride canprevent the formation of hot spots in the catalyst and serves to improvethe performance, especially selectivity, of the catalyst greatly. Thepreferred amount of the reaction modifier is several ppm (by weight) toseveral tens of ppm.

The following Examples and Comparative Examples illustrate the presentinvention.

EXAMPLE 1

Na₂ CO₃ (26.9 g) was dissolved in 1 liter of water, and 1 kg of analpha-alumina carrier (ring-like with a size of 8 φ×3 φ×8 mm; surfacearea 0.5 m² /g; pore volume 0.4 ml/g) was immersed in the solution. Theexcess of the solution was removed by permitting it to drip, and thenthe impregnated carrier was dried on superheated steam at 140° C. for 15minutes.

Separately, 248 g of AgNO₃ was dissolved in 1 liter of water and 148 gof potassium oxalate (K₂ C₂ O₄.H₂ O) was dissolved in 1 liter of water.The solutions were mixed, and heated to 60° C. in a water bath to obtaina white precipitate of silver oxalate. After filtration, the precipitatewas washed with distilled water to remove potassium from theprecipitate. Separately, 21.7 g of 1,3-propanediamine and 79.1 g ofethylenediamine were dissolved in water to prepare 200 ml of an aqueoussolution. With ice cooling, the aqueous solution was added little bylittle to the silver oxalate precipitate to prepare a silveroxalate-amine complex solution. The solution was then mixed with 40 mlof an aqueous solution containing 1.49 g of barium nitrate and 0.234 gof cesium chloride. The mixture was transferred to a rotary evaporator.The above alpha-alumina carrier impregnated with Na₂ CO₃ and dried wasadded to the above mixture, and an impregnating operation was performedat 50° C. with rotation. The pressure was reduced (100 mmHg) at theearly stage of the impregnating operation. The pressure was returned tonormal atmospheric pressure, and 5 minutes later, the impregnatedalpha-alumina carrier was taken out. The alpha-alumina carrier washeated with superheated steam at 200° C. for 10 minutes by passing thesteam at a rate of 2 m/sec to prepare a catalyst in accordance with thisinvention.

The amounts of Ag, Na, Ba, Cs and Cl deposited were 13.5%, 0.4%, 670ppm, 158 ppm and 42 ppm, respectively.

FIG. 1 (solid line) shows the heating curve obtained at this time. It isseen from the heating curve that the impregnated carrier was rapidlyheated with superheated steam to the boiling temperature of water.Presumably, water was evaporated from the entire surface of the carrier,and a uniform distribution of the catalyst ingredients could beobtained. Thereafter, until the carrier is heated to 200° C., theevaporation of water and the decomposition of the silver complex tookplace continuously.

FIG. 2-A is a scanning electron micrograph (magnification 10,000×) ofthe catalyst prepared. The silver grains which were fine and uniformwere deposited on the inner surfaces of the pores of the carrier. Thedistribution of silver shown in FIG. 2-A was maintained over the entirecatalyst particles, and the average diameter of the silver grains was0.15 micron. Almost all of the silver grains had a diameter within therange of 0.05 to 0.3 micron. The catalyst had a BET surface area of 0.95m² /g.

FIG. 3 (solid line) shows a distribution of the loading of silver in thecatalyst particle. The loadings of Ag, Cs and Na in a portion measuringup to 6% by weight from the outside surface of the catalyst particletoward its interior were 15.0%, 177 ppm, and 4350 ppm, respectively. Theloadings, I, I_(Cs) and I_(Na), in the interior of the catalyst particlein a portion measuring at least 60% by weight from the outside surfacetoward the inside were 13.0%, 134 ppm, and 3500 ppm. Accordingly, I,I_(Cs) and I_(Na) were calculated to be about 0.87S, 0.76S_(Cs) and0.835S_(Na). This shows that the individual ingredients were uniformlydistributed from the outside layer to the innermost layer of eachcatalyst particle.

The catalyst was crushed to a size of 4 to 9 mesh, and 5 ml of it wasfilled in a steel reaction tube having an inside diameter of 20 mm. Areaction gas composed of 30% by volume of ethylene, 8% by volume ofoxygen, 2 ppm of vinyl chloride and the remainder being nitrogen waspassed through the reaction tube under a pressure of 18 kg/cm² -G at anSV of 4000 h⁻¹. Immediately after the reaction gas was fed through thecatalyst layer, the catalyst began to act. The reaction was carried outfor 1 week at a bath temperature of 212° C. The conversion of oxygen was40%, and the selectivity of ethylene oxide was 81.7%. After thereaction, the catalyst had a BET surface area of 0.84 m² /g. To maintainan oxygen conversion of 40% during a continuous operation for 1.5months, the bath temperature was raised by 2° C., but there was nochange in selectivity.

EXAMPLES 2-6

Example 1 was repeated except that barium nitrate was not added, and thetemperature of the superheated steam and the time of its passing, etc.during the final heating were changed as shown in Table 1. Table 1summarizes the I/S ratios of the resulting catalysts, and the results ofthe catalyst carried out as in Example 1. The loadings of Ag, Na, Cs andCl were 13.5% by weight, 0.4% by weight, 158 ppm, and 42 ppm,respectively. In the table, T₄₀ and S₄₀ show the bath temperature (°C.)at which the oxygen conversion was 40%, and the selectivity (%),respectively.

                  TABLE 1                                                         ______________________________________                                               Temperature × time ×                                       Ex-    flow rate of                                                           ample  superheated steam                                                                              I/S     T.sub.40                                                                             S.sub.40                               ______________________________________                                        2      200° C. × 10                                                                      0.82    213    81.4                                          min. × 2 m/s                                                     3      200 × 180 × 2                                                                      0.82      213.5                                                                              81.0                                   4      150 × 10 × 2                                                                       0.70    210    80.8                                   5      230 × 8 × 2                                                                        0.85    213    81.7                                   6      300 × 8 × 2                                                                        0.80    214    80.6                                   ______________________________________                                    

After the catalyst of Example 2 was used continuously for three months,T₄₀ was 218° C., and the S₄₀ was 80.8%,

EXAMPLES 7-8

Ag-CsCl-containing catalysts were prepared in the same way as in Example1 except that the same alpha-alumina carrier as used in Example 1 butnot having sodium carbonate supported thereon was used; that bariumnitrate was not added, and that the time during which superheated steamwas passed was changed as shown in Table 2. The amounts of silver andCsCl deposited were 13.5% by weight and 200 ppm, respectively.

Using the resulting catalysts, the same oxidation reaction of ethyleneas in Example 1 was carried uut. The results are also shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                Temperature × time ×                                      Ex-     flow rate of                                                          ample   superheated steam                                                                             I/S      T.sub.40                                                                           S.sub.40                                ______________________________________                                        7       200° C. × 5                                                                      0.75     215  80.0                                            min. × 2 m/s                                                    8       200° C. × 120                                                                    0.74     215  79.3                                            min. × 2 m/s                                                    ______________________________________                                    

COMPARATIVE EXAMPLE 1

This comparative example serves to clarify the difference between thecatalyst of the present invention and a catalyst prepared by ashort-time air heating method in accordance with another invention ofthe present inventors.

Example 2 was repeated except that the final heating was carried out bypassing heated air at 200° C. for 10 minutes at a flow rate of 21 m/secinstead of using superheated steam at 200° C. A catalyst having the samecomposition as in Example 2 was obtained. Its heating curve is shown bya broken line (control catalyst) in FIG. 1. As shown by the broken linein FIG. 1, the temperature rising rate was slower in the initial stageof heating than in the case of using superheated steam (the solid linein FIG. 1), and a constant rate period of drying at a temprature belowthe boiling temperature proceeded. It is presumed that in this state thedrying occurred mainly n the outside surfaces of the carrier particles,and therefore, the catalyst ingredients moved to the outside surfaces ofthe particles. Thereafter the decomposition of the silver salt complexproceeded.

FIGS. 2-B and 2-C show scanning electron micrographs of the resultingcatalyst. FIG. 2-B shows the state of the inner surfaces of the poresnear the outside surface, and FIG. 2-C shows the state of the innersurfaces of the pores in the interior portion. It is clearly seen fromthese photographs that there was a non-uniform distribution of Ag grainswithin the catalyst particle.

FIG. 3 (broken line) shows the distribution of Ag within the catalystparticle. S of Ag in a portion measuring up to 6% by weight from theoutside surface layer of the catalyst particle toward its interior was18.8%, and I in a portion measuring more than 60% by weight from theoutside surface toward the interior was 12%. Hence, I/S was about 0.64.The foregoing results show that in contrast to the superheated steamheating, Ag deposited on the carrier was distributed nonuniformly in airheating.

The same reaction as in Example 1 was carried out using the resultingcatalyst (Comparative Example 1). In the early stage of the reaction,T₄₀ was 217° C., and S₄₀ was 81.3%. After a continuous operation forthree months, T₄₀ was 227° C., and S₄₀ was 79.8%. It is seen that incomparison with the catalyst of Example 2, the range of temperature risewas large, and the decrease of the selectivity was remarkable.

COMPARATIVE EXAMPLE 2

A catalyst having the same composition as the catalyst of Example 6 wasprepared by repeating Example 6 except that heating for depositing Na₂CO₃ on the carrier was carried out by using air at 140° C. instead ofusing superheated steam at 140° C., and the final heating was carriedout for 2 hours by using heated air at 300° C. instead of usingsuperheated steam at 300° C. FIG. 2-D shows a scanning elecronmicrograph (magnification 10,000×) of the resulting catalyst. It is seenthat Ag grains were markedly agglomerated. The average diameter of theAg particle was 0.4 micron, and their particle size ranged from 0.15micron to 0.7 micron. The catalyst had a BET surface area of as low as0.53 m² /g. The I/S within the catalyst particle was about 0.60, and theI_(Na) /S_(Na) was about 0.22. Thus, it was confirmed that thedistributions of Ag and Na within the catalyst particle werenon-uniform. When this catalyst was used in the same reaction as inExample 1, its performance was very low as shown by an oxygen conversionof 8% and a selectivity of 72% at 250° C.

EXAMPLE 9

A catalyst was prepared in the same way as in Example 1 except that theloading of barium was changed from 670 ppm to 1700 ppm.

The performance of the catalyst was tested by using it in the samereaction as in Example 1 except that the reaction was carried out for 1week at a bath temperature of 213° C. The oxygen conversion was 40%, andthe selectivity of ethylene oxide was 81.6%. After the reaction, thecatalyst had a BET surface area of 0.80 m² /g. During continuousoperation for 1.5 months, the bath temperature was raised by 2° C. tomaintain an oxygen conversion of 40%, but the selectivity did notchange.

EXAMPLES 10-11

Example 9 was repeated except that the amount of Ba added and the sourceof Ba were changed as indicated in Table 3. The results obtained areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                  Catalyst   Ba          T.sub.40                                                                           S.sub.40                                Example   composition                                                                              source      (°C.)                                                                       (%)                                     ______________________________________                                        10        Ag: 13.5%  Ba(OH).sub.2                                                                              211  81.7                                              Cs: 158 ppm                                                                   Cl: 42 ppm                                                          11        Na: 0.4%   BaO.sub.2   211  81.8                                              Ba: 50 ppm                                                          ______________________________________                                    

In this catalyst system, the addition of even a small amount of Baevidently led to an increase in activity and selectivity.

What is claimed is:
 1. A catalyst for the production of ethylene oxidefrom ethylene, said catalyst comprising a porous carrier composed of amolded article of a refractory material and having thereon (i) silvergrains, (ii) as a cationic component, at least one alkali metal elementselected from the group consisting of lithium, sodium, potassium,rubidium and cesium, and (iii) as an anionic component, at least oneelement selected from the group consisting of fluorine, chlorine andbromine, wherein(A) silver is distributed on the outside surface of thecarrier and on the inner surfaces of the pores of the carrier, (B)silver grains distributed on the inner surfaces of the pores of thecarrier have an average diameter of 0.05 to 0.2 micron, (C) the loading(S) of silver on the outside surface layer of the catalyst and theloading (I) of silver on the innermost layer of the catalyst satisfy thefollowing formula (1):

    I≧0.7S                                              (1)

in which the said outside surface layer of the catalyst denotes aportion having a weight of about 5% by weight on average, in the rangeof about 4-6%, shaven an uniformly as possible from the outside surfaceof one catalyst particle toward its inner layer when the weight of onecatalyst particle is taken as 100%, and the said innermost layer of thecatalyst denotes an inner layer of the catalyst particle which is leftafter about 60% on the average, in the range of about 50 to 70%, hasbeen shaven off from the outside surface of the catalyst particle towardits inner layer as uniformly as possible, and (D) the loading (Sc) ofthe alkali metal element as the cationic component on the outsidesurface layer of the catalyst and the load (Ic) of the alkali metalelement as the cationic component on the innermost layer of the catalystsatisfy the following formula (2-1) or (2-2):

    Ic≧0.4Sc                                            (2-1)

in the case where the alkali metal is sodium or lithium,

    Ic≧0.6Sc                                            (2-2)

in the case where the alkali metal is potassium, rubidium or cesium, inwhich said outside surface layer of the catalyst and said innermostlayer of the catalyst are as defined above.
 2. The catalyst of claim 1in which the silver grains are present on the porous carrier in anamount of 5 to 20% by weight based on the entire catalyst.
 3. Thecatalyst of claim 1 in which(i) in the case where sodium, as the alkalimetal element, is present on the porous carrier, the total amount ofsodium is 50 ppm to 1% by weight, and in the case where lithium ispresent on the porous carrier, the total amount of lithium is 0.1 to 1%by weight, based on the entire catalyst, (ii) in the case wherepotassium and/or rubidium, as the alkali metal element, is present onthe porous carrier, the total amount of potassium and/or rubidium is notmore than 0.1% by weight based on the entire catalyst, and (iii) in thecase where cesium, as the alkali metal element, is present on the porouscarrier, the amount of cesium is from 10 ppm to 0.5% by weight based onthe entire catalyst.
 4. The catalyst of claim 1 in which as a secondcationic component, in addition to at least one of said alkali metalelements, barium is present on the porous carrier.
 5. The catalyst ofclaim 4 in which the silver grains are present on the porous carrier inan amount of 5 to 20% by weight based on the entire catalyst.
 6. Thecatalyst of claim 4 in which(i) in the case where sodium, as the alkalimetal element, is present on the porous carrier, the total amount ofsodium is 50 ppm to 1% by weight, and in the case where lithium ispresent on the porous carrier, the total amount of lithium is 0.1 to 1%by weight, based on the entire catalyst, (ii) in the case wherepotassium and/or rubidium, as the alkali metal element, is present onthe porous carrier, the total amount of potassium and/or rubidium is notmore than 0.1% by weight based on the entire catalyst, and (iii) in thecase where cesium, as the alkali metal element, is present on the porouscarrier, the amount of cesium is from 10 ppm to 0.5% by weight based onthe entire catalyst.